Centrifugal fans for commercial applications typically include a fan wheel that is rotated by a constant-speed motor. The fan wheel includes a generally flat, circular back plate and a plurality of spaced-apart blades arranged near the radial edge of the back plate. The blades protrude outwardly from the plane of the back plate. As the back plate is rotated by the motor, the blades rotate about the rotational axis of the fan wheel, which axis is perpendicular to the center of the back plate.
The blades are arranged to define a central cavity within the fan wheel. The blades are shaped and angled so that, as the fan operates, air is drawn into the cavity along a direction generally parallel to the fan wheel rotational axis and forced radially outwardly from the cavity. The fan wheel is contained in a housing that directs the outlet air into the distribution system to which the fan is connected.
A generally frustum-shaped inlet cone is mounted adjacent to the cavity of the fan wheel. The inlet cone is shaped to direct ambient air into the fan wheel cavity in a manner that maintains a substantially laminar air flow stream.
Application requirements for fan systems sometimes specify that the outlet air volume should be variable to match system air volume requirements that vary over time. In this regard, it is usually advantageous to reduce inlet air flow into the fan, whenever the system requirements permit such a reduction, so that the power requirements of the fan motor may be reduced. The power requirements (hence, the operating cost) of the fan motor decrease with the decrease of air flow into the fan wheel. Put another way, reducing the air flow into the fan "unloads" the fan wheel, thereby reducing the load driven by the fan motor.
One known technique for controlling inlet air flow into the fan wheel employs a control disk that is mounted for movement along the rotational axis of the fan. The disk can be moved into a "closed" position against the inlet cone for completely occluding inlet air flow to the cavity of the rotating fan wheel. The disk may be driven away from the inlet cone into a fully "open" position so that the fan wheel is completely exposed to the inlet air flow. The mechanism for moving the disk may be controlled for positioning the disk at any location between the full open and closed positions.
A disk-type inlet air flow control mechanism as just described is disclosed in U.S. Pat. No. 4,808,068, issued to Asbjornson et al. In that patent, the mechanisms described for moving the disk are a motor-driven lead screw arrangement, and a linear actuator/drive screw assembly. Those motor- and lead screw-driven mechanisms are mounted in a manner such that the force that can be applied by the motor or the linear actuator must exceed the maximum force generated by the air pressure acting on the disk as the air is drawn into the cavity of the fan wheel. This force (i.e., the net pressure acting on both sides of the disk) is greatest as the disk approaches the closed position while the fan wheel is rotating. The cost of motors and actuators that have enough power to reliably overcome this maximum force adds a substantial increment to the overall cost of the fan system, especially for applications that require large fan wheels that generate large forces on the control disk.